Recently, biologists have discovered a toolkit of developmental genes that are widely conserved across animal phyla. Although the roles of these genes in determining body plans of major animal groups have been established, their role in microevolution is less clear. One such conserved set of genes governs the development of the vertebral column. In most vertebrate species, the number of vertebrae is remarkably constant. In humans, for example, deviation from the modal number of seven cervical vertebrae is associated with neural problems and cancer. In snakes, however, individuals in a single population can differ by 10-15 vertebrae. Vertebral number is heritable within populations, population differences have a genetic basis, and population and species differences are a consequence of selection rather than drift. Is this relatively rapid evolution due to natural selection acting on allelic variants of ancient developmental genes? To answer this question, I will test models of the selective processes responsible for vertebral number diversification within and among garter snake species. I will then design primers for a set of candidate genes involved in vertebral development, sequence these loci in a nested phylogenetic design, and test for selection at the nucleotide level. Evidence for variation and selection will indicate a largely unstudied microevolutionary role for these loci. Conversely, lack of variation and a negative selection result would suggest that other loci or regulatory pathways are involved in vertebral development. [unreadable] [unreadable] [unreadable]